Electric wires produced by coating with polyester-based insulating varnishes followed by baking are widely used in general or domestic electric appliances because of their excellent electrical insulation properties, heat resistance and mechanical properties. Since, however, such polyester-based insulating varnishes have a high molecular weight and are hydrophobic, they are usually dissolved in phenol- or cresol-based organic solvents which are expensive and harmful. Attempts have heretofore been made to make polyester resins water-soluble.
One of them is a procedure in which polyvalent (at least trivalent) carboxylic acid is incorporated into the main chain as a water-soluble portion. This method, however, causes plasticization of the main chain, thereby not only reducing thermal and mechanical characteristics, but also bringing a disadvantage that the resulting polyester resin gels before its molecular weight reaches a certain level at which it exhibits characteristics sufficiently comparable to those of solvent-type polyester-based insulating varnishes, because of the equivalency in reactivity of the carboxyl group.
Another procedure is to sufficiently increase the OH/COOH ratio and limit the molecular weight to a certain extent so that the skeleton of the resin is rendered water-soluble before it becomes hydrophobic. In this case, however, the resulting polyester-based varnish provides an insulating coating which is inferior in thermal and mechanical properties to that of the organic solvent-type polyester-based insulating varnishes, because of its low molecular weight.